1. Field of the Invention
The invention relates to a technical field of pulse width modulation and, more particularly, to a method of designing a random pulse width modulation (RPWM) inverter with unwanted harmonic elimination.
2. Description of Related Art
When the RPWM technique is applied to an inverter, the harmonics of an output voltage of the inverter are suppressed. In addition, the suppressed harmonics of the output voltage are expansively distributed over the entire spectrum. Therefore, the RPWM technique, unlike the sinusoidal pulse width modulation (SPWM) technique that produces special high harmonics nearby a multiple of switching frequencies, does not collect the harmonic energy at certain frequencies and can be used to drive a motor or generate AC sources, thereby improving the problems caused by the harmonics on running the motor or generating AC sources.
The features of a PWM inverter are determined by the output waveform. Programmed PWM inverters can have the optimal output waveforms and cancels the unwanted harmonics upon the objective functions or cost functions for various applications. The programmed PWM inverters have the advantages of reducing the torque vibration of an AC motor, the current ripples and the losses, and canceling the unwanted harmonics. However, for such inverters, solving the non-linear equations to obtain the output waveforms is required.
The programmed PWM inverters typically reduce more low-order harmonics. However, such a way produces great harmonic components nearby the side bands, and the harmonic components can cause electromagnetic interferences (EMIs) and acoustic noise. In addition, the switching frequencies of such inverters are increased with the increased number of reduced low-order harmonics, and thus the switching losses are increased. Moreover, even the noises are reduced, the output waveforms not produced by an RPWM are fixed, which can produce the specific-frequency, periodic noises as well.